(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to a method and apparatus for measuring particulate contamination in an enclosed chamber during wafer processing.
(2) Background of the Invention and Description of Previous Art
A conventional way of detecting particle contamination in a wafer processing chamber is by using an in-situ particle monitor (ISPM) installed in the exhaust line of the chamber. The working principle of most ISPM involves counting the number of particles that pass through a light or laser beam. Such a method is not effective if there is ‘blind spot’ in the chamber which prevents efficient flow of purge gas to the exhaust line. Particles tend to build up in these ‘blind-spots’ while the ISPM still detects and reports a low particle count. The build-up of particles in these blind spots may eventually contaminate wafers processed in the chamber. FIG. 1 show the cross-section of a typical annealing chamber 10 having an upper housing 12 containing an array of quartz/halogen lamps 14 in a reflector unit 15 and a lower body 16, isolated from the upper body by a seal 17. The lower body 16 houses a rotatable support ring 18 having an opening with a recess onto which a wafer 20 is placed for processing. In use annealing gases are flowed through the chamber through the gas ports 25. The chamber 10 may also be fitted with a vacuum port (not shown). The support ring 18 is typically formed of silicon carbide and is supported on a fused quartz cylinder 22. In use, the support ring is rotated at a slow speed by a motor (not shown). Particulates, generated by the drive mechanism, in particular by the action of the bearings, build up in the regions 24 between the support ring and the lower body and can, in time, migrate onto the wafer's backside causing contamination thereon. Left in place, these particulates can migrate onto the wafers top side causing yield losses. Yield losses can occur not only in the chamber wherein the particles are initially deposited but, by transfer of the contaminated wafer to subsequent processing tools. Both the subsequent tools and the wafers processed therein can become contaminated. Migration of wafer backside contamination to device areas can occur as a result during subsequent processing or during physical examination of wafer backside. The regions 24 are considered as ‘blind spots’ because they cannot be observed by conventional optical sensors such as ISPMs. Blind spots occur in most wafer processing chambers.
The annealing chamber shown in FIG. 1 is used here primarily to illustrate regions in a typical processing chamber where particulates accumulate and which are not easily observable in real time using the traditional optical means. Other chamber types which have mechanical devices which operate during processing, in particular wafer rotation mechanisms, may include CVD (chemical vapor deposition) chambers, RIE (reactive ion etching) and plasma etching chambers, ion implant chambers, and PVD (physical vapor deposition) chambers (evaporators). Controlling particle contamination on the backside of a wafer by early detection is important to effective yield management.
Wafer backside contamination can also cause yield problems associated with lithographic depth of focus. If the wafer, more specifically the wafer's exposure field, cannot be maintained in a fully planar position during lithography exposure due to wafer backside contamination, the lithography tool cannot expose the entire wafer uniformly causing wafer to be scrapped. Therefore, it would be advantageous to have a method and apparatus for real-time monitoring of particle contamination in a wafer processing chamber that will take into account the presence of particle build up in ‘blind-spots’ of the chamber and thereby providing an early signal of the onset of backside contamination problems before expensive yield losses occur.
Hiatt, et. al., U.S. Pat. No. 5,963,315 discloses a method for measuring and monitoring backside contamination on semiconductor wafers while the wafer is still in the processing tool. The wafer is removed from the chuck, which secures it during processing, by a robotic arm. The arm then positions the wafer over a laser-detector arrangement and the exposed backside is scanned for particulate contamination. At completion of the scan, the robotic arm transports the wafer to the next location which may be into another processing chamber or into a load-lock for removal from the processing tool. Aqui, et. al., U.S. Pat. No. U.S. 5,347,138, provide real-time particle monitoring in a processing tool by passing a externally focused laser beam through a shielded plasma to an external detector, using windows in the tool. Moriya, et. al., U.S. Pat. No. 6,115,120, Ashan, et. al., U.S. Pat. No. 5,481,357, Bonin, et. al., U.S. Pat. No. 5,943,130, and Harwell, et. al., U.S. Pat. No. 6,032,544 teach similar procedures wherein external optical beams are focused and pass through flowing gases within a process chambers. Optical sensors then determine particulate intensity by measuring the light scattering caused by the particulates in the beam. Baier, U.S. Patent Application Publication No. 2004/0173310 A1 detects and monitors sidewall flaking in a plasma chamber by measuring the intensity of light scattered by polymer film materials deposited on the wall surface of plasma chamber. Chanayem, U.S. Pat. No. 5,271,264 discloses a method of in-situ particle monitoring in a plasma etching chamber by positioning an ISPM downstream of the chambers vacuum pump.
Koury, et al., U.S. Pat. No. 5,814,733 discloses a method of using an accelerometer for real time monitoring of vibrations in a wafer probe tester produced by various mechanical components such as fans and motors, which cause probe contact errors and otherwise produce inconsistent and erroneous electrical test results.
Although the final objective of the cited prior art is similar, all of the cited methods are limited to detecting particulates in regions of the processing tool which an optical beam/sensor arrangement can observe in real time. None of the optical methods are capable of real time monitoring of particle build up in the so-called ‘blind-spot’ regions of a processing chamber which cannot be practically fitted with optical detection equipment. It would therefore be desirable to have a non-optical method and apparatus for real-time monitoring of particle contamination buildup in a wafer processing chamber. The method and apparatus taught by the present invention not only accomplishes this goal but is also simpler, cost effective, and easier to implement than the optical methods and is capable of sensing and reporting conditions in blind spot regions.